Implantable medical devices are available to provide therapies for restoring normal cardiac rhythms by delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, delivers an appropriate therapy, such as cardiac pacing, upon detecting an abnormal rhythm. Cardiac pacing is delivered in response to the absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
In modern implantable cardioverter defibrillators, the physician programs the particular anti-arrhythmia therapies into the device ahead of time, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher energy cardioversion pulse may be selected. For an overview of tachycardia detection and treatment therapies reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al.
Detection of tachycardia or fibrillation may also trigger the storage of the sensed intracardiac electrogram (EGM) for a period of several seconds such that the EGM signals leading up to and during a detected arrhythmia episode are available for downloading and displaying on an external programmer or other device for analysis by a physician. Such analysis aids the physician in monitoring the status of the patient and the patient's response to delivered therapies. Occasionally, cardioversion or defibrillation therapies are delivered when the patient does not feel symptomatic. In such cases, the ICD may inappropriately detect a tachycardia or fibrillation episode that does not exist and deliver an anti-arrhythmia therapy when it is not needed. Inappropriate arrhythmia detection is generally caused by oversensing, which can be defined as the sensing of events other than the one P-wave and/or the one R-wave occurring during each normal sinus cardiac cycle. Oversensing of both cardiac and non-cardiac events can result in inappropriate arrhythmia detection by the ICD if the detected rate due to oversensing falls into an arrhythmia detection zone. Oversensing can also impact the timing of pacing algorithms and in some causes may withhold life sustaining pacing therapy for pacemaker dependent patients when oversensing occurs over a long period of time (e.g. myopotentials). Cardiac oversensing refers to oversensing of cardiac events such as far-field P-waves, T-waves, or R-waves that are sensed twice and are therefore “double-counted”.
Anti-tachycardia pacing therapies delivered during normal sinus rhythm can potentially induce an arrhythmia in some patients, making the delivery of a therapy in response to inappropriate arrhythmia detection highly undesirable. Therefore, what is needed is a method and apparatus for reducing instances of oversensing.